characteristics to aerogels. As far as water treatment is concerned, xerogels would be

preferred over aerogels due to the higher mechanical strength of the xerogel.

 Curing. Calcination or curing is the final heat treatment step in producing a

mechanically strong gel. This thermal treatment also affects several characteristics of the

silica gel. With increased heat treatment, the BET (Brunauer, Emmett, and Teller

equation) surface area will decrease due to sintering (Papirer, 2000) and the dehydration

(around 180-200 C) and subsequent dehydroxylation (above 2000 C) of the surface of

the silica, resulting in the formation of siloxane bridges (Holysz, 1998). The final product

resulting from all these steps hydrolysiss, condensation, gelation, aging, drying, curing) is

a gel containing a relatively monodisperse pore size and displaying specific

characteristics associated with the conditions it experienced during processing.

 Effect of catalyst. The two alkoxides most often used are tetramethoxysilane

(TMOS) and tetraethoxysilane (TEOS). TMOS has the advantage of rapid hydrolysis

under a variety of conditions, but the toxic methanol produced can be hazardous to the

eyes and lungs. Conversely, TEOS is hydrolysis rate limited (Brinker and Scherer, 1990),

but produces less toxic ethanol during the reaction. To increase the hydrolysis reaction

rate of TEOS, an acid or base can be added as a catalyst. However, the nature and

concentration of the catalyst will affect the characteristics of the gel. For example, the

addition of hydrofluoric acid will rapidly increase the rate of gelation as well as increase

the pore size (Powers, 1998).

 Other important parameters to consider in the production of a gel are H20 to Si

molar ratio (called the "R" factor) and the aging and drying schedules. A high R ratio will

promote more rapid hydrolysis, for the concentration of water affects the rate of